Risers for the connection of an oil or gas well to an oil rig are well known in prior art. Normally a riser may extend several hundred meters or even several kilometers from the well up to an off-shore rig. A riser is formed by a plurality of tubes, normally with a length of approximately 10 meters, which are interconnected to each other by means of pipe couplings that are specifically adapted to the requirements and conditions of this kind of application. During normal operation of the riser, the latter is used for the purpose of conducting oil or gas from the well to the rig. However, in cases when maintenance work is required anywhere along the riser or in the region of the top of the well, for example in the region of the well head and the blow out preventer, the riser may be used for the purpose of transporting necessary maintenance equipment down to the maintenance site. These are, however, all well known properties and functionalities of off-shore risers.
Normally, the tubes by which the riser is assembled are delivered as plain tubes to the off-shore plant in which they are to be used. There, specific coupling elements are attached the ends of each tube such that the latter can be effectively connected to each other in order to actually build the riser. Preferably, coupling elements that define bayonet couplings are used for this purpose. Thereby, the coupling elements that are attached to tubes may in fact form part of the tubes. For example, a tubular coupling element, forming a female part in a pipe coupling, may be welded to one end of one of the above-mentioned tubes, thereby forming an elongation of said tube. A tubular coupling element, forming a male part in said pipe coupling, is welded to the other end of said tube. Tubes equipped with such tubular coupling elements are then connected to each other, one at a time, with interacting male and female parts.
Pipe couplings of prior art comprise an annular sealing element which is provided between and overlaps opposing ends of first and the second tubular elements that are to be interconnected by a pipe coupling. The annular sealing element has a T-shaped cross-section, wherein said sealing element comprises a first portion that forms a stem of said T-shaped cross section and extends in a radial direction of the sealing element and defines the outer periphery thereof, and a second portion formed by two opposing flanges that form the cross-bar of the T-shaped cross section and define the inner periphery of the annular sealing element. A first of said flanges overlaps and is in sealing contact with an inner circumferential surface of the first tubular element, and the second flange overlaps and is in sealing contact with an inner circumferential surface of the second tubular element. During operation, the elevated pressure inside the tubular elements will press the flanges of the sealing element towards the inner periphery of the respective tubular element that they overlap.
However, the pipe couplings, typically of bayonet type, are of such design that they permit a certain displacement of the interconnected tubular elements in relation to each other, in axial direction as well as by means of biasing. Thereby, there will also be a slight displacement between the contact surfaces of the sealing element and the corresponding surface of the respective tubular element and, as a consequence thereof, certain wear on said surfaces.
Sealing elements of prior art, as defined above, have generally flat sealing surfaces by means of which they abut an inner periphery of the tubular element with which they interact in a sealing manner. In order to adopt motions such as biasing of adjacent tubular elements in relation to each other, the flanges of the sealing elements of prior art may be designed such that they flex somewhat when subjected to forces caused by such motions. However, it has been found that there is still considerable wear on said sealing surfaces, which in the long run will lead to less good sealing capacity.